The black liquor is discharged from a pulping (removal of lignin) process of wood substances. The origin of the black liquor is pulping liquor, and since the pulping liquor uses sodium hydroxide to dissolve lignin in water and further uses sodium sulfide to promote removal of lignin, the black liquor is a harmful alkaline liquid substance containing hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide generated by hydrolysis of sodium sulfide as well as carbohydrates, organic acids and resins other than lignin.
In a conventional black liquor treatment method, this liquid substance is concentrated using a large-scale multi-effect vacuum evaporator method to increase the concentration of a solid portion, and the concentration is further increased by oxidization by air, sodium sulfide and an effluent of sulfate salt are added to this black liquor and then burned. After that, sodium carbonate, sodium sulfide, which are inorganic chemicals, are recovered from the ash, and burning energy is used for water feed heating of a boiler, but much of it is discharged to the air.
And the sodium carbonate is dissolved in water, calcium hydroxide is converted to sodium hydroxide, and the sodium hydroxide is used as the pulping liquor again to promote recycling.
However, the equipment cost of the measures in the conventional art is large, malodorous substances leak to the outside of the facilities, and a large quantity of water is used. And since most of lignin is burned and discharged to the air as an exhaust gas, required treatment costs are extremely large.
The unique odor felt in the vicinity of plants for paper manufacturing/pulp and paper manufacturing in our country is caused by sulfur compounds such as dimethyl sulfide. Since a large quantity of water is used, it is discharged after use and emission of exhaust gas is extremely large, there is a demand for black liquor treatment in a method friendly to the environment.